17:15 〜 18:30
[SSS26-P09] Focal Mechanisms and Seismicity in the Region of Induced Earthquakes of Song Tranh Dam, Vietnam
#Quoc Cuong Nguyen, James Mori (DPRI, Kyoto Univ.)
キーワード：Song Tranh Dam, VietNam, Focal Mechanism , Induced Earthquake, Velocity structure
Vietnam is located in South East Asia and bounded by the Pacific and Mediterranean-Himalaya seismic belts on its eastern, western and southern sides, respectively. The dynamic tectonic processes in this region cause the territory of Vietnam and adjacent areas to have intensive differential movement, making the regional tectonic structure very complicated. The tectonics have led this territory to have moderate seismic activity and complicated geological structures, such as the Lai Chau-Dien Bien fault zone, Red River fault zone, and others. Southern Vietnam was considered to be a region with low seismicity, compared to the North. However, the sequence of earthquakes that occurred at Song Tranh Dam during the last several years surprised many scientists because the southern region of Vietnam was not expected to have major tectonic activity. This region where many induced earthquakes are now occurring is associated with the filling of a new reservoir. There have been four M4 earthquakes (maximum earthquake was 4.7 in November, 2012), so it is one of the most active induced earthquakes examples in the world. It is important to determine the strong motion attenuation relations for this area since damaging earthquakes may be expected in the near future. We collect and process data from 5 seismic stations around Song Tranh dam, include more than 300 events larger than 1.5 and more than 2000 seismic waveforms to determine arrival times and locate the earthquakes in the Song Tranh dam region. In this study we use time domain analyses to determine focal mechanisms. We use software of Dreger and Ford (2011) modified for the Song Tranh Dam region. Induced earthquakes processed by this software include events withmagnitudes larger than 3.5 and recorded on 4 or more stations.
We also compare our results with mechanisms for tectonic earthquakes in the region (Hung Nhuong Tavi and Tra Bong faults). The results show a difference in focal mechanism between tectonic earthquakes and induced earthquakes which may be related to the increased fluid pressure from filling of the reservoir. To confirm this result, we will need to process the many smaller events with magnitude less than 3.0, which have occurred around Song Tranh Dam.
We used a genetic algorithm method to estimate the local velocity structure. We applied this method to determine a layered model for the Song Tranh dam region. Our results obtained a new 1D model of 7-8 layers. The shallow P wave velocity of 4.6 km/s is slower than 5.9 km/s for previous studies in northern VietNam. For a deeper layers from 6 to 12 km, P wave velocity becomes larger, 5.4 km/s – 5.9 km/s. The Vp/Vs shows relatively higher values of 1.75-1.77 for the depth around 12 km. When layer thickness changes from 21 km to 28 km, the P wave velocity increases and changes from 6.5 km/s to 7.3 km/s, however, Vp/Vs ratio decreases from 1.77 to 1.67. Finally, the depth of the Moho surface changes from 28 to 35 km and the P wave velocity changes from 7.8 to 8.2 km/s, with Vp/Vs value of about 1.78. Earthquakes still occur at Song Tranh dam (a recent M3.3 occurred on August,26th 2015), and more than a thousand earthquakes with magnitude less than 1.5 have not yet been processed. We continue to update the seismic analyses with information from smaller earthquakes to improve our results.
We also compare our results with mechanisms for tectonic earthquakes in the region (Hung Nhuong Tavi and Tra Bong faults). The results show a difference in focal mechanism between tectonic earthquakes and induced earthquakes which may be related to the increased fluid pressure from filling of the reservoir. To confirm this result, we will need to process the many smaller events with magnitude less than 3.0, which have occurred around Song Tranh Dam.
We used a genetic algorithm method to estimate the local velocity structure. We applied this method to determine a layered model for the Song Tranh dam region. Our results obtained a new 1D model of 7-8 layers. The shallow P wave velocity of 4.6 km/s is slower than 5.9 km/s for previous studies in northern VietNam. For a deeper layers from 6 to 12 km, P wave velocity becomes larger, 5.4 km/s – 5.9 km/s. The Vp/Vs shows relatively higher values of 1.75-1.77 for the depth around 12 km. When layer thickness changes from 21 km to 28 km, the P wave velocity increases and changes from 6.5 km/s to 7.3 km/s, however, Vp/Vs ratio decreases from 1.77 to 1.67. Finally, the depth of the Moho surface changes from 28 to 35 km and the P wave velocity changes from 7.8 to 8.2 km/s, with Vp/Vs value of about 1.78. Earthquakes still occur at Song Tranh dam (a recent M3.3 occurred on August,26th 2015), and more than a thousand earthquakes with magnitude less than 1.5 have not yet been processed. We continue to update the seismic analyses with information from smaller earthquakes to improve our results.